1. Field of the Invention
The present invention relates to microwave excited lamps and to microwave gaskets therein.
2. Description of the Prior Art
FIGS. 1 and 2 illustrate a schematic diagram of a prior art microwave excited lamp 10 of the type manufactured by the Assignee of the present invention. The microwave lamp 10 has a magnetron 12. The magnetron 12 is rated with maximum sustained output power of at least 1,000 watts and typically at least 2,000 watts. The magnetron 12 includes a housing 14 of a conventional design including heat radiating fins (not illustrated) thereon to dissipate heat generated therein. The magnetron 12 typically oscillates at a frequency, such as 2.45 GHz, to produce the aforementioned output power. The magnetron 12 includes an antenna 18 which radiates microwaves within a microwave structure 20. The microwaves are coupled to the UV electrodeless bulb 24 within a reflector 22 to cause the emission of UV light therefrom. The microwave structure 20 is manufactured from a metallic material, such as aluminum or stainless steel and contains the reflector 22. The reflector 22 has a curved reflective surface 23 which reflects the UV wavelengths of the light produced by the UV bulb 24 through a metallic mesh 28 to a target (not illustrated). The metallic mesh 28 contains the microwaves within the reflector 22 but is transparent to the UV rays. The microwaves are coupled into the reflector 22 by slots 30.
It should be understood that the magnetron 12 of FIG. 1 has been omitted from FIG. 2 in order to illustrate the backside surface 34 of the reflector 22 extending along the microwave structure 20 and the prior art usage of silver-plated brass wire mesh microwave gaskets 40 which provide microwave seals against aluminum structures within the microwave structure. The arcuate section 31 set off by dotted lines is a surface area where it has been discovered that at least one standing wave microwave current maxima exists during operation.
A pair of rails 32 connect the back surface 34 of the reflector 22 to opposed side walls 36 of the microwave structure 20 to provide a support structure for the reflector. The microwave structure 20 is electrically conductive and comprises aluminum. The microwaves transmitted from the antenna 18 flow on the surface 34 of the reflector 22 and through the slots 30. The microwaves are prevented from leaking below the rails 34 by the silver-plated brass wire mesh microwave gaskets 40 and outside the end walls 44 by silver-plated brass wire mesh microwave gaskets 42 which each are of identical construction, and are manufactured from multiple wire filaments. The microwave gaskets 40 provide a microwave tight seal between the back surface 34 of the reflector 22 and the opposed side walls 36 of the microwave structure 20. The microwave gaskets 42 seal the microwaves from leaking beyond the end plates 44.
The current maxima in location 31 has been discovered to occur approximately in the middle third of the microwave structure 20 therewith discussed below regarding FIG. 3. Oxidation of the silver-plated brass wire mesh gasket material occurs due to the presence of ozone, UV light, and heat during lamp operation. The progressive oxidation process increases the amount of contact resistance between the gasket and the reflector and the rails.
The gaskets 40xe2x80x2 (new) and the gasket 40xe2x80x3 (after use) of FIG. 3 are identical to the gasket 40 of FIG. 2 and are wire manufactured-from silver-plated brass filaments. Silver plating was chosen because of low electrical resistance. The new silver gasket 40xe2x80x2 is not oxidized and, as a result, provides the low electrical resistance between the gasket and aluminum of the reflector 22 and the rails 32 to the substantial alternating microwave current flow occurring within the area 31 over the back surface 34 of the aluminum reflector 22, the surfaces of gasket 40, the surface of the longitudinally extending aluminum rails 32, and the surface of the opposed aluminum walls 36 of the microwave structure 20. Alternating current circulates in a loop back and forth on the surface of the aforementioned parts and causes a voltage drop across the contact resistance between the silver-plated brass wire mesh gasket 40 and the aluminum reflector 22 and the aluminum rails 32.
The silver-plated brass wire mesh gasket 40xe2x80x3 has been used for 3,059 hours in a lamp of the design of FIGS. 1 and 2 for generating UV light. It has been discovered by the inventor that the substantial oxidation of the silver-plated brass wire mesh gasket 40xe2x80x3 is a result of the presence of heat, ozone and UV light. It has been further discovered by the inventor that the substantial oxidation in the vicinity of the area 31 increases the electrical contact resistance inherent in the electrical contact between the dissimilar materials of the aluminum used in the manufacture of the reflector 22 and rails 32 and the silver-plated brass of the gasket 40. The increase in contact resistance caused by the oxidation of the silver plating in the vicinity of the current maxima in area 31 is so substantial that a sealing failure of the gaskets 40 and a heat related failure of the reflector 22 of FIGS. 1 and 2 occurs. The highly oxidized state of the gasket 40xe2x80x3 produces an increased contact resistance causing localized heating at the gasket interface with the back surface 34 of the reflector 22. The heating destroys the reflector requiring both the gasket 40 and the reflector 22 to be replaced to maintain operational status of the lamp 10. The rails 32 may also be locally damaged by the heat in the vicinity of the area 31.
The present invention is an improved microwave excited lamp and gasket for use therein having a substantially longer life than the aforementioned prior art silver-plated brass wire mesh gasket and associated reflector of aluminum in electrical contact therewith. In accordance with the invention, a light reflector in a microwave excited lamp, the gasket and optionally at least one other metallic part of the microwave structure all comprise a common metallic material, such as elemental aluminum or stainless steel which substantially eliminates contact resistance. Since materials containing the same metal do not have appreciable contact resistance, the aforementioned problem of increased contact resistance caused by oxidation in the area of a current maxima inside the microwave structure which flows on the surfaces of the reflector, gasket, and at least one other part is eliminated. The metallic material used in the gasket, reflector and optionally any other parts may comprise, may consist essentially of or may consist of aluminum or stainless steel. In accordance with the invention, the reflector, microwave gasket, and optionally the at least one other part, do not have to be manufactured from identical metallic materials with it being within the scope of the invention to have different alloys comprising the metallic material, such as elemental aluminum or different alloys of stainless steel containing different percentages of ingredients.
Test results show that the failure of the prior art gasket 40xe2x80x3 illustrated in FIG. 3 does not occur with the invention as a result of the reflector, gasket and optionally at least one other part comprising the same metallic material, such as elemental aluminum or stainless steel.
A microwave excited lamp in accordance with the invention includes a microwave source; a microwave structure including a reflector comprising a metallic material and containing a microwave excited bulb which emits light in response to coupling of microwave power thereto from the microwave source; and at least one microwave gasket which provides a microwave tight seal between a surface of the reflector and at least one other part of the microwave structure with the gasket comprising the metallic material; and wherein during operation of the lamp a microwave current maxima flows on a surface of the gasket and the reflector. Each microwave source may be rated at at least 1000 watts and may be at least 2000 watts. The reflector and the gasket may comprise, may consist essentially of, or consist of elemental aluminum or alloys thereof. The reflector and the gasket may comprise, may consist essentially of, or consist of stainless steel of different compositions. The microwave gasket may comprise wire. The reflector may be curved and have an axis extending along the microwave structure; and the microwave structure may comprise opposed parts and a pair of members extending along the reflector which are respectively joined to the opposed parts and to the reflector and for each member a pair of microwave gaskets respectively may provide a microwave tight seal between the member and an adjacent one of the opposed parts and the member and a back surface of the reflector. The opposed parts may be opposed walls of the microwave structure.
In a microwave excited lamp, including at least one microwave source, a microwave structure comprising a metallic material to which microwaves are coupled from the microwave source, the microwave structure including a reflector comprising the metallic material and containing a microwave excited bulb which emits light in response to coupling of microwave power thereto from the microwave source, and at least one microwave gasket which provides a microwave tight seal between a surface of the reflector and at least one other metallic part of the metallic microwave structure, and wherein during operation of the lamp, a microwave current maxima flows on a surface of the reflector and the gasket, a gasket in accordance with the invention includes the metallic material. Each microwave source may be rated at at least 1000 watts and may be rated at at least 2000 watts. The reflector and the gasket may comprise, consist essentially of, or consist of elemental aluminum. The reflector and the gasket may comprise, may consist essentially of, or may consist of stainless steel. The gasket may comprise wire of multiple filaments.